This invention pertains to high performance composite flywheels, and more particularly a flywheel with a rim and integral hub for a high speed, large energy storage flywheel energy storage system with significantly reduced costs. The flywheel includes a two ring, standard modulus carbon fiber rim that is press-fit together and pressed over a tapered solid steel hub. The flywheel rim rings are filament wound carbon fiber in an epoxy resin matrix that allows increased operating temperature and radial strength of the rims with simultaneous manufacturing ease and use of low cost constituent materials.
Modern flywheel energy storage systems convert back and forth between a spinning flywheel""s rotational energy and electrical energy. A flywheel energy storage system includes a flywheel, a motor/generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor/generator converts energy between electrical and mechanical, and the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low pressure enclosure to minimize aerodynamic losses that would occur from operation in the atmosphere, while low speed systems can be operated at atmosphere.
Several types of flywheel designs are used for energy storage; they can be classified into three groups based on their design attributes: low performance industrial, high performance industrial and aerospace. Low performance industrial flywheels are normally constructed of steel and all the energy stored in the flywheel is stored in the rotational inertia of the low cost spinning steel flywheel. The performance of existing steel flywheels in terms of tip speed and energy per unit weight is low, primarily because they are generally limited to tip speeds around 200 m/sec or less. However, the advantages of low performance industrial (steel) flywheels are that they are relatively simple and low cost.
The second group of energy storage flywheels, high performance industrial, uses composite materials for increased flywheel performance. Filament wound glass fibers in conjunction with low cost standard modulus carbon fibers in an epoxy matrix are typically used to form the energy storage rim. The flywheels are typically designed so that the rim stores most of the flywheel""s energy, usually more than 90%. Therefore, the goal of designers has been to minimize the cost of the rim by using the lowest cost materials and manufacturing techniques for construction of the rim. However, the use of the lowest cost fibers for the rim results in an undesirable attribute: a rim with significant radial growth when spun to high speed. The large growth is due to the lower elastic modulus of the low cost fibers. For example, glass fiber has an elastic modulus of about 10-13 million pounds/square inch (msi) compared to the elastic modulus of standard modulus carbon fiber of about 30-39 msi. A lightweight hub is used to couple the rim to the shaft. The hub must be designed to match the substantial growth of the inner periphery of the low cost composite rim, so that the rim will not grow radially away from and become separated from the hub, but instead remain connected to the hub at high speed. Most flywheel designs use machined aluminum hubs and less frequently hubs made from composite materials. Some hubs contain thin bending elements that allow the hub to grow with the rim. Metallic hubs are typically made from aluminum instead of steel because of the higher coefficient of thermal expansion. This higher coefficient of thermal expansion facilitates shrink fit installation of the hub and allows for approximately twice as much precompression of the hub so that it can follow the large rim growth at high speed. In some designs, the space available inside the flywheel rim inner diameter can be used for integral placement of the motor/generator or bearing system.
The design goal for high performance industrial flywheels is improvement over the performance of steel flywheels while maintaining relatively low cost. These high performance flywheels have higher tip speeds than the old low performance industrial flywheels, ranging from 500 to 1000 m/sec. The benefits of this higher rotation speed capability include storage of more energy, reduced bearing loads due to lighter flywheel weight, and the use of smaller or more powerful motor/generators due to the higher operating frequency.
Aerospace flywheels are designed for use in satellites and other space applications including space station power storage. The high launch costs for lifting objects into space make the design goals of this group of flywheels unique compared with industrial flywheels. The relative importance of the cost of the actual flywheel is insignificant compared to its performance, that is, the energy to weight and energy to size ratios. These flywheels are constructed of mostly if not all composite materials. Designs however focus on using the highest strength, more expensive intermediate modulus carbon fibers (40-50 msi). In many cases, the designs also concentrate most of the material comprising the flywheel to close to the outer diameter. This increases the inertia and energy storage of the flywheel while minimizing its weight. A radially preloaded press-fit construction in conjunction with use of the highest strength carbon fibers further allows for higher tip speeds, usually over 1000 m/sec, hence more energy storage per unit weight. Hubs, if metallic, are made small to reduce the flywheel weight. Flywheels used for defense applications such as mobile rail gun compulsators have many of the same or similar design and cost objectives and should be included in the aerospace category.
Of the composite material flywheels, various designs have been proposed and constructed, dating back to the 1970""s. Many designs for these high-speed energy storage flywheels included filament wound composite rings which are made of either glass and or carbon fibers in an epoxy matrix. These rings are usually wound with the fibers in the hoop direction and sometimes with a small amount of axial direction reinforcement added. Radial direction fibers are not added with filament winding. Such filament wound rings have the inherent advantage of very high hoop direction strengths, which are needed to match the very high hoop stresses generated during rotation. One drawback to the use of hoop wound composite rings for the rim portion of a high-speed flywheel is the inherently low radial tensile strength resulting from the absence of fiber reinforcement in that direction. It is therefore desirable to construct flywheels that operate in radial compression instead of radial tension. Because the radial direction stresses in a rotating filament wound ring are controlled by the non-dimensionalized radial thickness of the ring (ratio of ID to OD), such rings must be made radially thin to reduce the radial tensile stresses generated. Because a single ring must be made very thin (ratio of ID to OD≈0.8) so that it does not fail at a prematurely low rotational speed, the ring becomes less effective for energy storage.
To increase the effective ring thickness and hence the energy storage capacity of composite rims, a common approach for high performance industrial flywheels is to construct the rim of several different material rings. The rim is designed with the material having the lowest ratio of elastic modulus to density (hereinafter referred to as xe2x80x9cspecific stiffnessxe2x80x9d) at the inner diameter. This is usually glass fibers. The material used in the rim rings surrounding the inner ring is progressively higher specific stiffness materials (usually standard modulus carbon fibers) toward the outer diameter. Materials with higher specific stiffness will grow less than materials with lower specific stiffness when subjected to high-speed rotation. Therefore, the hybrid material composite rim with radially graduated specific stiffness precludes generation of unacceptable radial tensile stresses during rotation because the inner ring or rings grows into the outer ring or rings. In many cases, the thick rim actually goes into self-generated radial compression when spun, thereby allowing safe and reliable operation.
To take advantage of the self-generated radial compression of the multiple fiber rim design, the rim is constructed using lower modulus fibers at its inner diameter. This usually means that the rim inner diameter is made of glass fibers that have a lower modulus than all carbon fibers. Using standard modulus carbon fibers instead for the inner diameter rim using intermediate modulus and high modulus carbon fibers in the outer rims is considered to be an undesirable design approach because of the very high cost and lower strength of high modulus carbon fibers. Using glass fibers for the rim inner diameter allows the inner ring to grow sufficiently to allow self-generated radial compression. However, using glass fibers in the flywheel rim increases the difficulty of maintaining secure connection of the rim to the hub at high rotation speeds. The inner diameter growth of such rims becomes large at high speed, making design of hubs that remain connected to the rim difficult. To match the high radial growth of the rim, hubs are commonly machined out to produce spokes or curved bending elements to allow the hub to grow radially with the rim. Significant machining of the hub along with the using of aluminum for the hub material for added shrink-fit precompression inherently reduces the weight and hence the energy stored by the hub in high performance industrial flywheels and increases the cost.
Aerospace flywheels achieve both higher rotation speeds and energy per unit weight than high performance industrial flywheels by using more costly construction. Instead of using single wound multiple material composite rings for self-generated radial compression, aerospace flywheels typically achieve radial compression solely through precompression or interference assembly. All of the rings are typically constructed of high strength carbon fiber. By assembling the rings together with a radial interference between each ring, the rings can be fully driven into radial compression at zero speed. When the rotor is spun to high speed, the radial compression between the rings lessens. At failure speed, the pressure between two or more rings goes into tension and the rings can separate.
Interference fits can be created three ways: thermal shrink fits, high-pressure resin injection and press-fits. Thermally shrink-fitting a smaller ring inside a larger one can be done for light interference fits. However, because the composite rings are anisotropic in nature, unacceptable internal stresses can be generated when rings undergo large changes in temperature. The low coefficients of thermal expansion of the carbon fiber rings also further reduces the level of interference that can be achieved with this method. The use of high-pressure resin injection method preloads the composite rings by the addition of a bonding agent. The bonding agent is injected in the radial gap between concentric rings at high pressure and is then cured in place. Although this method has been shown capable of generating large radial precompression, it is only typically used for very long rotors where the tooling and process complexity are justified. For these reasons, aerospace flywheels generally use press-fit construction to achieve radial precompression.
Press-fit assemblies produce radial precompression by having composite rings with tapered inner and outer diameter surfaces. The inner ring has a mean outer diameter that is larger than the mean inner diameter of the outer ring. The rings initially slide telescopically together some amount, usually more than 50% together. A large hydraulic press is then used to axially force the rings all the way together, completing the assembly and driving the rings into radial compression. Epoxy is applied to the sliding surfaces prior to press-fitting to serve as both a lubricant aiding in the sliding and also as a bonding agent to keep the rings together after assembly. Because the flywheel rim is used almost exclusively for the energy storage, the rim is typically constructed of between 3 to 10 rings for sufficient storage and to allow the denser hub to be made adequately small and lightweight. More expensive, high strength intermediate modulus carbon fibers are used in the outer rings of the rim to further maximize the aerospace flywheels"" performance.
In addition to the actual fibers used and overall flywheel construction technique chosen, the resin used to manufacture a composite flywheel is equally important. The resin system must have adequate temperature capability, high toughness and strength, and properties conducive to low cost, high speed manufacturing, all with low material costs itself Many resin types currently exist and have been used for manufacture of composite materials. Each has advantages and disadvantages but none to date has been shown have favorable attributes in all the areas required for manufacturing of low cost, high performance commercial flywheels.
The disclosed invention is a value-optimized flywheel having a unique combination of flywheel elements elements to achieve a new, significantly lower cost, high performance flywheel. The invention uses a solid steel flywheel hub and a flywheel rim made of two rings press-fit together and pressed onto the hub. The two rings are made of standard modulus carbon fiber wound in an epoxy matrix. The invention uses a cost integrated design approach, considering the cost per unit energy storage of the hub and rim combination. Instead of focusing solely on the cost of the rim, the flywheel cost per unit energy storage capacity is considered for the flywheel as a whole. A complete fully stressed low cost solid steel flywheel hub is combined with a more expensive all carbon fiber press-fit rim such that the hub and the rim each store roughly equal amounts of energy. The new combination inherently allows for the lowest cost high performance composite flywheel by allowing simultaneous efficient stress loading and energy storage of each part of the flywheel. The energy stored in the flywheel with the carbon fiber rim is more than double the energy stored in the steel flywheel hub alone, and the use of the solid steel hub significantly reduces the cost per energy of the high-speed composite flywheel. The resulting flywheel can operate at tip speeds of at least 700 m/sec.
The hub, because it is solid, can operate at both its maximum allowable radial and hoop direction tensile stresses, and each of the two standard modulus carbon fiber rings operates near their maximum allowable hoop stresses. A solid hub, besides storing significant energy, generates equal radial and hoop stresses, whereas a machined hub would significantly increase one of these stresses depending on the design. This allows a solid hub to be made larger. The solid hub also significantly reduces machining costs over conventional flywheel hubs. The radial stresses in the composite rings are kept in compression throughout the entire speed range through the use of the press-fit. By using only carbon fiber and avoiding the use of glass fiber in the rim, the higher elastic, modulus of the all-carbon fiber rim limits growth at high speed so that the rim and hub stay connected at high rotation speeds. By employing the maximum allowable radial interference pressures between the two rings and the inner ring and hub (roughly 15 ksi for hoop wound composite rims), the use of only standard modulus carbon fiber is adequate to keep both the rings in radial compression and connected to the hub at high speed. The installation of the hub by press-fitting instead of a thermal shrink fit permits much higher initial interference between the hub and rim, allowing the rim and hub to stay connected at high speed. The composite rim is more expensive than conventional high performance flywheel rims because of the use of more standard modulus carbon fiber instead of glass fibers (Standard Modulus carbon fiber costs about five to ten times more than the cost of E-glass fiber by weight.). However, because the steel hub, costing about the same as E-glass by weight, is also used for energy storage, the total cost per energy is less. At the same time, the performance of the invention is equally high, with a tip speed over 700 m/sec.
The new composite flywheel rims are manufactured with a new resin and curing agent mixture that facilitates construction of flywheels is described. Flywheels in accordance with this invention can be manufactured with the new resin mixture with the highest speed capability while having adequate temperature stability, easy manufacturing, low residual stresses and low costs. Because the properties of resins usually involve tradeoffs, the resin mixture invention is optimized for the unique requirements specific for flywheels and their manufacture to achieve a unique balance of properties.